The numerous examples of "crosstalk" between signal transduction pathways reported in the biochemical literature seem to imply a general common response of cells to different stimuli, even when these stimuli act initially on different cascades. This contradicts our knowledge of the specificity of action of extracellular signals in different cell types. This discrepancy is explained by the restricted occurrence of crosstalks in any cell type and by several categories of cell specificity mechanisms, for instance, the specific qualitative and quantitative expression of the various subtypes of signal transduction proteins, the combinatorial control of the cascades with specific sets of regulatory factors and the compartmentation of signal transduction cascades or their elements.